The present invention relates generally to jet aircraft turbine engines and, more specifically, to a hush kit for use on an existing engine and airframe.
In view of noise restrictions placed upon the use of aircraft in certain areas and at certain times, a need has existed and continues to exist for quieter aircraft engines. There is a significant need for a method to modify conventional engines on aircraft that presently are in service, since aircraft with such engines often cannot be used economically, because of noise restrictions. Further, jet engines currently in production must satisfy noise restriction requirements.
In December, 1969, the U.S. Federal Aviation Administration (FAA) promulgated specific noise level regulations for aircraft. Similar noise standards were prescribed by international civil aviation organizations, for example, ICAO Annex 16. Thus, it has become imperative for airframe and engine manufacturers and others to take noise considerations into account in designing, building and using jet aircraft.
The major airframe manufacturers and others have undertaken substantial research efforts toward developing retrofit kits for existing airplanes and engines to attempt to meet Federal Aviation Regulation (FAR) 36 noise level requirements. However, kits which would meet the noise level requirements, particularly the more stringent Stage 3 requirements, without degrading performance, increasing fuel consumption and unreasonably increasing costs, were difficult to achieve. This was particularly difficult for the Pratt & Whitney JT8D engines used with the 737 airplanes made by Boeing Aircraft Company.
Studies which had been conducted showed that noise radiates from a bypass, fan jet engine, such as the JT8D engine, in several directions. High frequency fan noise radiates both forward through the air inlet and afterward through the exhaust ducts. Low frequency jet noise generally radiated rearwardly. At low engine thrust, the high pitch wine of the fan is more pronounced while at high engine thrust, a low pitch jet rumble is more noticeable. Each noise component must be addressed in order to meet the FAA noise restriction requirements.
The past efforts to design or retrofit aircraft engines to suppress noise levels have not been fully acceptable or successful. Many of the past designs and retrofits simply have not provided a meaningful noise reduction at a reasonable cost and without a performance penalty.